Which tranny is in 1990 Ford F350 Diesel?

I've got a 1990 F350, 1 ton dual wheel diesel, with a 3 speed tranny... I just was wondering, does it have a C6 like most other heavy duty fords? or is it some special diesel tranny?

should be special

I also have a 90 f350 dually diesel (was a u-haul) and its a c6..it did have HD clutches though. The shop I took it to put regular clutches in it and it only lasted a few months.